And this is evident from the (extortionate for me) amount of purple fringing and distortion especially at the wide end of the zooms.

I'm not sure this isn't a product of how large and optically uncompromised the 4/3s zooms were.

rumor has it that Olympus took existing FF designs and added wideconverters (for some lenses)... rumor though.

I don't think anyone suggested that they took existing designs. It's quite true though that the f/2 zooms (and the 7-14) have a 0.5x focal reducer at the back (like a built in 'Speed Booster') which means that the front end must be a design with 2x the focal length.